1. Field of the Invention
This invention in general relates to the field of laser printing systems and, more specifically, to an optical printhead which produces gray-scale images comprising printing elements of more than one size.
2. Description of the Prior Art
The use of lasers, especially semiconductor lasers, in optical printheads wherein pulses of optical radiation are utilized to produce an image or graphics, is well known. In a typical application, an image is produced by the scanning of the optical radiation across a suitable receptor medium. By way of example, the image may comprise alphanumeric characters or a graphical image formed either in gray scale or in color.
A laser printing system may be configured to produce a gray-scale image by imaging pixels comprised of dark and light regions, such as in halftone recording methods. As disclosed in the prior art, a halftone pixel can be comprised of one or more clusters of individual print or display units commonly referred to as "pels." Although pels are commonly provided in a single fixed size, the resolution of the printed or displayed image may be improved by using pels of more than one size. For example, in U.S. Pat. No. 5,355,303 "Printing apparatus," issued to Feria et al. there is disclosed a printing apparatus in which an individual 90 .mu.m.times.90 .mu.m gray-scale pixel is comprised of both 30 .mu.m wide pels and 5 .mu.m wide pels.
In some applications, the laser printing system is used to image onto a binary thermal imaging medium such as that described in, for example, U.S. Pat. No. 5,200,297 "Laminar thermal imaging mediums, containing polymeric stress-absorbing layer, actuable in response to intense image-forming radiation," issued to Kelly, which discloses a laminar thermal imaging media comprising a pair of sheets at least one of which is transparent and responding to incident thermal energy, such as that provided by optical laser radiation. When using a binary imaging medium, the process of imaging requires that, to produce a pel, the intensity of the imaging laser beam at the medium be above a preestablished threshold but remain below a maximum level so as not to produce burnout.
Typically, the output distribution of an imaging laser beam is Gaussian. To achieve desired intensity at a particular pel size, the beam must be focused upon the medium within a certain tolerance. Moreover, because of different spatial distributions and thermal considerations, the exposure requirements will differ for different pel sizes. One approach is to use a different current driver for each type of pel used in the imaging pixel. This, however, adds additional cost and complexity to the device.
While the art describes a variety of devices that utilize laser devices for printing, there remains a need for improvements that offer advantages and capabilities not found in presently available instruments, and it is a primary object of this invention to provide such improvements.
It is another object of the present invention to provide a printhead laser driver current source which provides an output waveform suitable for optimally producing pels of more than one size.
It is another object of the invention to provide such a current source which will lessen the tolerance requirements on the focusing mechanisms.
Other objects of the invention will be obvious, in part, and, in part, will become apparent when reading the detailed description to follow.